The present invention relates generally to phase shifting circuits and more particularly to a phase shifting circuit of the type in which the amount of phase shift may be controlled or varied.
There are many existing circuits which provide a shift in the phase of an input signal. Perhaps the simplest and best known types of these circuits include various combinations of inductive, capacitive and resistive components which may be fixed, to provide a fixed shifting of phase, or variable for allowing a changing of the phase shift. A number of these circuits result in at least some degree of attenuation. Known circuits satisfy most applications for which a phase shifting is desired. They do, however, suffer generally from certain defects including a lack of stability resulting from component value variation due to aging, temperature changes, etc. More importantly however, known circuits for this purpose generally suffer from the lack of being readily adjustable from a remote location. That is, while it may be relatively easy for an operator to adjust a potentiometer or a variable capacitor or inductance by physically turning a knob or similar adjustment mechanism, the ability to perform this function remotely is difficult. Quite often, the remote adjustment of such a circuit requires the use of selsyn systems, or the like, which renders the overall system not only expensive but also difficult to control with accuracy.
The need for an accurate, remotely controllable phase shifting circuit or network recently became apparent in the control of subsynchronous resonance in a turbine/generator system. Subsynchronous resonance occurs in a turbine/generator system due to the fact that some transmission lines have a resonance which is at a lower frequency than the generating system output frequency. This line resonance frequency, when fed back into the turbine/generator system results in a "beat" frequency which sets up torsional forces in the shafts interconnecting the various masses of the turbine/generator system. These torsional forces have been known to actually cause breakage failure of the shafts. It has been found that these subsynchronous resonant frequencies tend to be of particular values and are especially pronounced, in their effects, at certain regions within the turbine/generator system. It has also been found that, by employing suitable phase shifting techniques in the control of the generator, these problems can be lessened. While the present invention found initial application in the solution of the subsynchronous resonance problem, it will be apparent from the following description that the invention has general applicability.